No more going unnoticed

Had the satellite shot down by the US navy last week crashed on Earth, it would, of course, have had impact. By and large, things are not so simple in the public and non-profit sector. There, the trick - all the more urgent now that spending is tightening - is to show impact in the absence of direct, visual evidence.

Private companies do not need to worry: their impact is assumed in sales figures, cash flow, profit. A charity that trades can measure its revenue, too. But what matters above and beyond its obligations under the statement of recommended practice is its - and the Charity Commission's - sense of public benefit.

Impact assessment has become the cry. When it reported recently, the Accounts Commission - the Scottish body responsible for monitoring value for money - warned councils to pay more attention to outcomes. They must demonstrate what they are about, especially when it comes to their impact on citizens. Across the public sector, the message is the same: show and tell what difference you are making.

It's a tune being hummed even in the House of Commons, a place notoriously resistant to modern management. The specialist backbench committees have just produced their annual reports, and they mostly make an effort to address their impact. This is not easy. Some of the reports fall into the trap of pretending inputs are outputs. The health committee records the number of site visits made by its members, to Ottawa, Nashville and Amiens among other destinations (to look at electronic patient records). That of course is an input.

But some inputs are simultaneously outputs. MPs, like many others in the public sector, do procedure - and the process itself may have intrinsic value. Requiring Alan Johnson to give evidence is an input that is also an output: the committee's impact lies in questioning the health secretary and other office holders in public. Its impact could be stiffening the sinews of democratic accountability.

At its simplest, impact is what the billiard balls do to one another in the example used by the philosopher David Hume to illustrate causality. The Treasury, unsurprisingly, is all for measures of impact of this kind. If you can't show that x has a demonstrable effect on y (teacher numbers and pupil attainment is one of the more controversial examples), then the case for spending more on x is weaker.

The trouble is that there are entire classes of public sector activity that are nigh impossible to measure in any straightforward way. Sir Ronnie Flanagan's report on the police toyed with the link between falling crime and increased police numbers, but no one really believes that the two are connected in a way that can be explained except over many footnoted pages, hedged with qualifications. But the public persists in its belief that bobbies on the beat have impact, and stop crime.

We are going to hear more arguments about staff numbers and impact, so it is important to get clear what is being, and what can be, measured. When it reported recently, the Audit Commission could say that deaths in fires have fallen to historically low levels and the fire service deserves the credit. Arrival speeds and equipment and training have a direct impact on mortality. But so does the painstaking business of making dwellings more fireproof, and that depends on multiple players: householders, manufacturers and local authorities as well as fire officers. Making a link between the preventive work that fire brigades are being encouraged to do and the number of fire incidents is going to be much harder to establish. Defending their corner, fire personnel will, not surprisingly, favour emergencies over humdrum preventive work.

We are moving into the new era of spending restraint, and already the shock troops of big defence spending are mobilising. Impact assessment is going to get even more fashionable. Because they can rarely produce profit and loss accounts, public bodies are sometimes excessively diffident about what they claim. But public managers must not be too quick to don their hairshirts.

Even without numbers you can still tell a convincing story. In its annual report, the Commons public administration committee robustly stuck up for the "real effect on people's lives" that MPs can have. It cited the campaign to back up the ombudsman in helping victims of the winding up of pension schemes as well as proposals on the distribution of power that had been, at least in part, accepted by ministers. Constitutional reforms, too, have impact - at least in the long run.

· David Walker is editor of Public, the Guardian's monthly magazine for public sector managers.